Studies are in progress to determine the role of properdin in connective tissue diseases. Data reveal that this protein is deposited in about 50 percent of clinically normal skin from patients with systemic lupus erythematosus. Studies of skin lesions from these patients, from patients with chronic discoid lupus erythematosus are continuing. Serum properdin levels in patients from whom biopsies of skin are obtained are measured by the "rocket" technique.